The present invention relates to a composition containing a support based on a cerium oxide, a zirconium oxide and a scandium or rare-earth oxide and use for exhaust gas treatment.
It is known that the nitrogen oxide (NOx) emissions in exhaust gases from motor vehicles are reduced, in particular, with the aid of three-way catalysts, which stoichiometrically use the reducing gases present in the mixture. Any oxygen excess leads to a pronounced deterioration in the catalyst""s performance.
However, certain engines, for example, diesel engines or lean-burn patrol engines, save on fuel but emit exhaust gases which permanently contain a large oxygen excess, of for example at least 5%. A standard three-way catalyst is therefore ineffective for the NOx emissions in this case. Furthermore, it has become imperative to limit NOx emissions owing to the tightening of motor vehicle emission standards which have now have been extended to these engines.
There is therefore a genuine need for an efficient catalyst to reduce NOx emissions for these types of engines and, generally, for treating games containing NOx.
As a type of catalyst which can meet this need, systems referred to as NOx traps have been proposed which are capable of oxidizing NO into NO2 and then of absorbing the NO2 thus formed. Under certain conditions, the NO2 is re-released then reduced to N2 by reducing species contained in the exhaust gas. These NOx traps are generally based on platinum. However, platinum is an expensive element. It would therefore be beneficial to provide a platinum-free system in order to reduce the costs of the catalysts.
The object of the invention is therefore to develop a catalyst which can be used as an NOx trap without necessarily using platinum.
To this end, the composition of the invention is characterized in that it comprises a support based on a cerium oxide, a zirconium oxide and an oxide of scandium or a rare earth other than cerium, and a supported phase based on manganese and at least one other element selected from the alkali metals, the alkaline-earth metals and the rare earths.
The invention also relates to a process for treating gases with a view to reducing nitrogen oxide emissions which is characterized in that a composition as defined above is used.
Other characteristics, details and advantages of the invention will become yet more fully apparent on reading the following description, as well as the various concrete but non-limiting examples intended to illustrate it.
The composition of the invention comprises a phase supported on a support.
The supported phase may more particularly correspond to two variants.
According to a first variant, this phase is based, further to manganese, on an alkali metal and/or an alkaline-earth metal. The alkali metal may more particularly be sodium or potassium. The alkaline-earth metal may more particularly be barium or strontium.
According to a second variant, the supported phase is based on manganese and at least one element selected from the rare earths.
Here, and for all of the description, the term rare earth is intended to mean the elements in the group consisting of yttrium and the elements in the Periodic Table having an atomic number of between 57 and 71 inclusive.
The rare earth may more particularly be elected from lanthanum, cerium, praseodymium, neodymium, europium, samarium, gadolinium or terbium. As an advantageous embodiment in the scope of this second variant, mention may be made of a supported phase based on manganese and praseodymium.
Lastly, it is entirely possible in the scope of the present invention to have a supported phase based on manganese and at least two other elements, one being a rare earth and the other being selected from the alkali metals and the alkaline-earth metals.
According to a particular embodiment, the composition of the invention can be obtained by a process in which at least one of the two elements manganese and potassium is supplied at least partially by potassium permanganate. It should be noted that a single element may be supplied by the permanganate, and only partially. Conversely, and preferentially, it is also possible to supply the two elements fully by the permanganate route. All of the variants between these two possibilities may be envisaged. This embodiment makes it possible to obtain compositions having high NOx adsorption capacities.
Another important characteristic of the composition of the invention is the nature of the support of the supported phase.
As indicated above, the support is based on a cerium oxide, a zirconium oxide and an oxide of scandium or a rare earth other than cerium. It is to be emphasized here, and for all of the description, that the invention also applies to any support based on cerium oxide, zirconium oxide and, as a third element, a combination of two or more oxides selected from scandium oxide and the rare-earth oxides.
The supports used are preferably those for which the cerium/zirconium atomic proportion is at least 1.
As a rare earth involved in the composition of the support, mention may more particularly be made of lanthanum, neodymium and praseodymium.
Use may also more particularly be made of the supports satisfying the overall formula CexZryMzO2 where M represents at least one element selected from the group comprising scandium and the rare earths other than cerium and where x, y and z satisfy the relationships 0 less than zxe2x89xa60.3, 1xe2x89xa6x/yxe2x89xa619 and x+y+z=1.
More particularly, x, y and z may satisfy the following relationships 0.02 less than z less than 0.2, 1 less than x/y less than 9, it being, more particularly still, possible for the last ratio to be between 1.5 and 4, these limits being inclusive.
According to a particular embodiment, the support is in the form of a solid solution. In this case, the X-ray diffraction spectra of the support reveal the existence of a single homogeneous phase within it. As regards supports which are richer in cerium, this phase actually corresponds to that of a crystallized cerium oxide CeO2 whose lattice parameters are shifted to a greater or lesser extent relative to a pure cerium oxide, thus reflecting the incorporation of zirconium and the other element (scandium and rare earths other than cerium) in the crystal lattice of the cerium oxide, and therefore the fact that a genuine solid solution is obtained.
According to a preferred variant of the invention, supports are used which are characterized by their specific surface at certain temperatures, as well as their oxygen storage capacity.
The term specific surface is intended to mean the BET specific surface determined by nitrogen adsorption according to the standard ASTM D 3663-78 established on the basis of the Brunauer - Emmett - Teller method described in the periodical xe2x80x9cThe Journal of the American Society, 60, 309 (1938)xe2x80x9d.
It is thus possible to use supports which have a specific surface after calcining for 6 hours at 900xc2x0 C. of at least 35 m2/g. This surface may more particularly be at least 40 m2/g. lt may, more particularly still, be at least 45 m2/g.
These supports may also have surfaces which are still considerable even after calcining for 6 hours at 1000xc2x0 C. These surfaces may be at least 14 m2/g, more particularly at least 20 m2/g and more particularly still at least 30 m2/g.
Another characteristic of the supports of this variant is their oxygen storage capacity. This capacity, measured at 400xc2x0 C., is at least 1.5 ml O2/g. It may more particularly be at least 1.8 ml O2/g and more particularly still at least 2 ml O2/g. In the best cases, this capacity my be at least 2.5 ml O2/g. This capacity is determined by a test which evaluates the capacity of the support, or of the product, to successively oxidize amounts of carbon monoxide injected with oxygen and to consume infected amounts of oxygen to reoxidize the product. The method employed is referred to as an alternative method.
The carrier gas is pure helium at a flow rate of 10 l/h. The injections are carried out by means of a loop containing 16 ml of gas. The amounts of CO are injected using a gas mixture containing 5% CO diluted in helium, while the amounts of O2 are injected by employing a gas mixture containing 2.5% O2 diluted in helium. The gases are analysed by chromatography using a thermal conductivity detector.
The amount of oxygen consumed makes it possible to determine the oxygen storage capacity. The characteristic value of the oxygen storage power is expressed in ml of oxygen (under standard temperature and pressure conditions) per gram of product introduced, and is measured at 400xc2x0 C. The measurements of oxygen storage capacity given here, and in the rest of the description, are taken from products pretreated at 900xc2x0 C. under air for 6 hours in a muffle furnace.
The supports of the composition of the invention can be prepared in known fashion. They may thus be obtained using a solid/solid reaction of the oxides or any other precursor such as carbonates. They may also be prepared by a wet route, that is to say by precipitation with a base of the salts of cerium, zirconium and the third element or elements, then calcining.
In the case of the preferred above-described variant employing supports defined by their specific surface and their oxygen storage capacity, the support may be obtained by a process in which a mixture is prepared in a liquid medium containing a cerium compound, a scandium or rare-earth compound and a zirconium solution, which is such that the amount of base needed to reach the equivalence point during an acid-base titration of this solution satisfies the molar ratio condition OHxe2x88x92/Zrxe2x89xa61.65; the said mixture is heated; the precipitate obtained is recovered and this precipitate is calcined.
This process will now be described more specifically.
The first step of this process consists in preparing a mixture in a liquid medium, generally in the aqueous phase, containing at least one cerium compound, at least one zirconium compound and a scandium or rare-earth compound. This mixture is prepared by using a zirconium solution.
This zirconium solution may be produced by acid attack on a reagent containing zirconium. Examples of suitable reagents include zirconium carbonate, hydroxide or oxide. The attack may be carried out using an inorganic acid such as nitric acid, hydrochloric acid or sulphuric acid. Nitric acid is the preferred acid, and the use of a zirconyl nitrate produced by nitric attack on a zirconium carbonate may thus most particularly be mentioned. The acid may also be an organic acid such as acetic acid or citric acid.
This zirconium solution must have the following characteristic. The amount of base needed to reach the equivalence point during an acid-base titration of this solution must satisfy the molar ratio condition OHxe2x88x92/Zrxe2x89xa61.65. Most particularly, this ratio may be at most 1.5, or yet more particularly, at most 1.3. In general, the specific surface of the product obtained tends to increase as this ratio decreases.
The acid-base titration is carried out in a way that is known. In order to carry it out under optimum conditions, a solution may be titrated which has been adjusted to a concentration of about 3.10xe2x88x922 mol per litre, expressed in terms of the element zirconium. Whilst stirring, a 1N sodium hydroxide solution is added to it. Under these conditions, the determination of the equivalence point (change of the pH of the solution) takes place cleanly. This equivalence point is expressed by the OHxe2x88x92/ZR molar ratio.
Particular examples of cerium compounds which may be mentioned include cerium salts such as cerium(IV) salts, such as nitrates or ammonium ceric nitrates for example, which are particularly suitable here. Ceric nitrate is preferably used. The solution of cerium(IV) salts may contain cerium in the cerus state, but it is preferable for it to contain at least 85% of cerium(IV). An aqueous solution of ceric nitrate may, for example, be obtained by reacting nitric acid with a hydrated ceric oxide prepared conventionally by reacting a solution of a cerus salt, for example cerus nitrate, with an ammonia solution in the presence of hydrogen peroxide. It is also possible to use a ceric nitrate solution which is obtained according to the process involving electrolytic oxidation of a cerus nitrate solution, as described in the document FR-A-2 570 087, and which may constitute an advantageous starting material.
It will be noted here that the aqueous solution of cerium(IV) salts may have some degree of initial free acidity, for example a normality varying between 0.1 and 4 N. According to the present invention, it is equally possible to employ an initial solution of cerium(IV) salts actually having some degree of free acidity, as mentioned above, as well as a solution which has been neutralized beforehand, more or less powerfully, by adding a base such as, for example, an ammonia solution or alkaline metal (sodium, potassium, etc.) hydroxides, but preferably an ammonia solution, so as to limit this acidity. It is then possible, in the latter case, practically to define a degree of neutralization (r) for the initial cerium solution by the following equation:   r  =            n3      -      n2        n1  
in which n1 represents the total number of moles of Ce(IV) present in the solution after neutralization; n2 represents the number of moles of OHxe2x88x92ions actually needed to neutralize the initial free acidity contributed by the aqueous solution of cerium(IV) salt; and n3 represents the total number of moles of OHxe2x88x92 ions contributed by the addition of the base. When the xe2x80x9cneutralizationxe2x80x9d variant is employed, the amount of base used will in all cases be necessarily less than the amount of base needed to obtain full precipitation of the hydroxide species Ce(OH)4 (r=4). In practice, this will involve a limitation to degrees of neutralization of no more than 1, and more preferably no more than 0.5.
The scandium or rare-earth compounds are preferably compounds which are soluble in water, in particular.
As examples of scandium or rare-earth compounds which can be used in the process in question, mention may for example be made of the salts of inorganic or organic acids, for example of the sulphate, nitrate, chloride or acetate type. It will be noted that the nitrate is particularly well-suited. These compounds may also be supplied in the form of sols. These sols can be obtained, for example, by neutralizing a salt of these compounds using a base.
The amount of cerium, zirconium and rare earths or scandium present in the mixture must correspond to the stoichiometric proportions required for obtaining a support with the desired final composition.
Once the initial mixture has been obtained in this way, it is then heated, according to the second step of the process in question.
The temperature at which this heat treatment, also referred to as thermohydrolysis, is carried out may be between 80xc2x0 C. and the critical temperature of the reaction medium, in particular between 80 and 350xc2x0 C., preferably between 90 and 200xc2x0 C.
Depending on the temperature conditions adopted, this treatment may be carried out either under normal atmospheric pressure or under a pressure such as, for example, the saturated vapour pressure corresponding to the temperature of the heat treatment. When the treatment temperature is chosen to be above the reflux temperature of the reaction medium (i.e. generally above 100xc2x0 C.), for example chosen between 150 and 350xc2x0 C., the operation is then carried out by introducing the aqueous mixture containing the aforementioned species into a sealed enclosure (closed reactor move commonly referred to as an autoclave), in which case the required pressure results merely from the heating of the reaction medium (autogenous pressure). Under the temperature conditions given above, and in an aqueous medium, it may thus be indicated by way of illustration that the pressure in the closed reactor varies between a value in excess of 1 bar (105 Pa) and 165 bar (165.105 Pa), preferably between 5 bar (5.105 Pa) and 165 bar (165.105 Pa). It is of course also possible to exert an external pressure, which is then added to that due to the heating.
The heating may be carried out either under an air atmosphere or under an inert gas atmosphere, preferably nitrogen.
The treatment time is not critical, and may thus vary within wide limits, for example between 1 and 48 hours, preferably between 2 and 24 hours.
At the end of the heating step, a solid precipitate is recovered which can be separated from its medium by any conventional solid/liquid separation technique, for example filtration, settling, drying or centrifuging.
It may be advantageous to introduce a base, for example an ammonia solution, into the precipitation medium after the heating step. This makes it possible to increase the yields with which the precipitated species is recovered.
It is also possible to add hydrogen peroxide in the same way, after the heating step.
The product, as recovered, may then be washed with water and/or aqueous ammonia, at a temperature between room temperature and the boiling point. In order to remove the residual water, the washed product may lastly, if appropriate, be dried for example in air, this being done at a temperature which may vary between 80 and 300xc2x0 C., preferably between 100 and 150xc2x0 C., the drying being continued until a constant weight is obtained.
It will be noted that it is, of course, possible for a heating step as described above to be repeated one or more times, identically or differently, after recovery of the product and optional addition of the base or hydrogen peroxide, in which case the product is returned to a liquid medium, in particular in water, and for example heat treatment cycles are carried out.
In a last step of the process, the recovered precipitate is then calcined, after optional washing and/or drying. According to a particular embodiment, after the thermohydrolysis treatment and optionally after returning the product to a liquid medium and an additional treatment, it is possible to dry the reaction medium obtained directly by spraying.
The calcining is carried out at a temperature of generally between 200 and 1200xc2x0 C., and preferably between 300 and 900xc2x0 C. This calcining temperature must be sufficient to convert the precursors into oxides, and is also chosen on the basis of the future working temperature of the support and while taking into account the fact that the specific surface of the product becomes commensurably lower as the calcining temperature employed increases. For its part, the calcining time can vary within wide limits, for example between 1 and 24 hours, preferably between 4 and 10 hours. The calcining is generally carried out under air, but calcining carried out, for example, under an inert gas is clearly not to be ruled out.
The supported phase may be deposited on the support in a known way. The procedure used may involve an impregnation method. A solution or slip of salts or compounds of the elements in the supported phase will thus firstly be formed.
Examples of salts which may be chosen include salts of inorganic acids, such as nitrates, sulphates or chlorides.
It is also possible to use salts of organic acids, and in particular salts of saturated aliphatic carboxylic acids or salts of hydroxycarboxylic acids. Examples which may be mentioned include formates, acetates, propionates, oxalates or citrates.
The support is then impregnated with the solution or slip. After impregnation, the support is optionally dried, and is then calcined. It should be noted that it is possible to use a support which has not yet been calcined prior to the impregnation.
The supported phase may also be deposited by atomizing a suspension based on salts or compounds of the elements of the supported phase and of the support.
It may be advantageous to deposit the elements of the supported phase in two steps. Thus, in the case of supported phases based on manganese and potassium and manganese and praseodymium reciprocally, the manganese then the potassium may advantageously be deposited in the first case, and the praseodymium then the manganese in the second case.
As indicated above, for the particular embodiment which applies to the case in which the supported phase comprises manganese and potassium, at least one of the elements manganese and potassium may be supplied at least partially by potassium permanganate.
It should lastly be noted that it is possible in the scope of the present invention for at least one of the elements of the supported phase to be introduced into the support during the actual preparation of the latter.
The levels of manganese, alkali metals, alkaline-earth metals and rare earths can vary in wide proportions. The minimum proportion is that below which NOx adsorption activity is no longer observed. They may in particular be between 2 and 50%, more particularly between 5 and 30%, these levels being expressed as atomic % with respect to the sum of the elements of the support and the elements relevant to the supported phase.
The compositions of the invention, as described above, are in the form of powders but may optionally be shaped to be in the form of granules, balls, cylinders or honeycombs of variable sizes. The compositions may thus be used in catalytic systems comprising a wash coat having catalytic properties and based on these compositions, on a substrate of, for example, the metallic or ceramic monolith type.
The invention also relates to a process for treating gases with a view to reducing nitrogen oxide emissions employing the compositions of the invention.
The gases which can be treated by the present invention are, for example, those output by gas turbines, thermal power station boilers or alternatively internal-combustion engines. In the latter case, these may in particular be diesel engines or lean-burn engines.
When they are brought into contact with gases which have a high level of oxygen, the compositions of the invention function as NOx traps. The term gases having a high level of oxygen is intended to mean gases having an oxygen excess with respect to the amount needed for stoichiometric combustion of the fuels and, more precisely, gases having an oxygen excess with respect to the stoichiometric value xcex=1. The value xcex is correlated with the air/fuel ratio in a manner which is known per se, in particular in the field of internal-combustion engines. Such gases are those from a lean-burn engine which have a level of oxygen (expressed by volume) of at least 2%, as well as those which have an even higher level of oxygen, for example gases from engines of the diesel type, that is to say at of least 5% or more than 5%, more particularly 10%, it being possible for this level to lie between 5 and 20%, for example.
The compositions of the invention may be associated with complementary emission control systems, such as three-way catalysts, which are effective when the value of xcex is less than or equal to 1 in the gases, or alternatively in systems involving fuel injection or exhaust gas recirculation (EGR) for diesel engines. They may also be associated with NOx catalysts for diesel engines.
The invention also relates to a catalytic system for treating gases with a view to reducing nitrogen oxide emissions, which gases may be of the type mentioned above and, more particularly, those having an oxygen excess relative to the stoichiometric value. This system is characterized in that it comprises a composition as described above.
Examples will now be given.